Effects of planting density and clones on the growth and wood quality of rubber tree (Hevea brasiliensis Muell. Arg.)

Rubberwood (Hevea brasiliensis) is a well-known plantation species in tropical areas. Wood properties are changed with genetic manipulation, silvicultural techniques, and site conditions. The main concern in wood utilization is the level of variation of wood properties. According the early researches, there are no adequate reports on how wood quality changes under different planting densities and clones? Understanding wood properties and its behavior under different conditions is very important for the evaluation of its products performance. Basic information on rubberwood of different clones and planting densities and their variation would be very valuable as an indicator for evaluating its suitability for diverse final uses. This study was carried out to characterize variation of anatomical, physical and mechanical properties as well as the tree growth parameters of a 9-year-old plantation grown wood in four different planting densities (PD); 500 (PD I), 1000 (PD II), 1500 (PD III), and 2000 (PD IV) trees ha-1 of two clones RRIM 2020 (I) and RRIM 2025 (II). This plantation was managed by Malaysian Rubber Board in the northeastern state of Terengganu, Peninsular Malaysia. Diameter at basal area (DBA), diameter at breast height (DBH) and clear bole height (BH) were measured to evaluate the effect of planting density on tree growth. The DBA and DBH showed significant negative correlation with planting density. The highest values of DBAs in both clones were 27.99 cm and 24.62 cm in PD I and the lowest values of 23.92 cm and 19.98 cm in PD IV. Likewise, the highest values of DBHs in both clones were 20.22 cm and 19.96 cm in PD I and the lowest values of 17.54 cm and 15.07 cm in PD IV. The BH revealed a significant and positive correlation with the planting density that showed an ascending trend from low to high planting densities. The highest values of 1023 cm and 1026 cm were in PD IV and the lowest values were 467 cm and 738 cm in PD I from both clones. The DBA and DBH in clone II were significantly smaller than clone I while the BH performance in clone II showed better results. The changes in fiber length and fiber wall thickness indicated a descending trend from low to high planting densities. Highest values of fiber length (1300 μm) in clone I was recorded in PD II and the lowest (1187 μm) in PD IV. The highest values of fiber length (1340 μm) in clone II was in PD I and the lowest (1272 μm) in PD III. Indefinite trends for fiber diameter and lumen diameter were seen in both clones. In clones I and II, the both features showed a decreasing trend until PD III and the increased in PD IV. Vessel density, vessel diameter, and vessel area in clone I from starting point of PD II showed an ascending trend to high planting densities. The PD I did not follow this trend. The highest values were in PD IV and PD I. In clone II, the trend of vessel density was ascending from the PD II. The highest value of vessel density was in PD IV and lowest value in PD II. Concerning vessel diameter and area, the trends were fluctuated. The highest values were recorded in PD I and PD III. Regarding the ray features, ray density (mm-2) and ray height showed an increasing movement from low to high planting densities. The trend of ray area indicated almost constant from low to high planting densities. The ray density in clone II was higher than clone I may relate with wood density. It was concluded that PD I and PD II can have higher quality wood in terms of longer fiber and thicker fiber wall. The mean air-dry wood density showed a descending trend from low to high planting densities. The highest values (0.59 and 0.64 g.cm-3) were revealed in PD I and the lowest (0.54 and 0.54 g.cm-3) in PD IV and PD III in both clones, respectively. The planting density had highest effect on wood density. Mean longitudinal shrinkage showed an increasing but insignificant trend from low to high planting densities in both clones. The tangential, radial and volumetric shrinkages decreased from low to high planting densities. Although they showed decreasing trend from low to high planting densities, there were significant differences between the different planting densities. The lowest values of shrinkages were recorded in PD IV for both clones. In general, the samples in clone II were more stable than clone I. The mechanical properties showed a descending trend from low to high planting densities. The highest values of MOR (87.18 Mpa and 98.22 Mpa) in both clones were in PD I, while the lowest values (83.10 Mpa and 85.43 Mpa) were evidenced in PD IV and PD III of clones I and II respectively. The MOE also showed a descending trend from low to high planting density. The compression parallel to grain followed a fluctuated trend that the highest value in clone I and clone II were in PD II and in PD IV respectively. The lowest values were in PD IV and PD III. The hardness in both clones had a little sway in PD II of clone I and PD III of clone II but it follows a decreasing trend. Among the two clones, PD I of clone II showed highest quality in strength. The shear parallel to grain like the hardness showed a decreasing trend toward high planting density but with a little sway in PD III of both clones. In general, the mechanical properties in clone II showed the better performance compared to clone I. On the whole, PD I and PD II shall produce higher quality of wood, with regards to longer fiber, thicker fiber wall and higher wood density. These factors greatly affect the wood quality. PD I of clone II exhibits the highest evidences being the best planting density.

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